Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet or continuous ink jet.
The first technology, drop-on-demand technology, provides ink droplets which impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the print head and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Commonly practiced drop-on-demand technologies use thermal actuation to eject ink droplets from a nozzle. With thermal actuators, a heater, located at near the nozzle, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble, increasing the internal ink pressure sufficiently for an ink droplet to be expelled. As is well known in the art, alternative methods of drop-on-demand droplet ejection use piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to vanLintel, on Jul. 6, 1993, bimetallic actuators, such as those disclosed by Lebens et al, U.S. Pat. No. 6,460,972, and electrostatic actuators, as practiced by Seiko Epson, Inc., disclosed in U.S. Pat. No. 6,474,784.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When a print is desired, the ink droplets are directed to strike a print medium. U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This early technique is known as binary deflection continuous ink jet. U.S. Pat. No. 4,636,808, issued to Herron et al., U.S. Pat. No. 4,620,196 issued to Hertz et al. and U.S. Pat. No. 4,613,871 issued to Katerberg disclose techniques for improving image quality in electrostatic continuous ink jet printing including printing with a variable number of drops within pixel areas on a recording medium.
Today's commercialized inkjet printers, whether of the drop-on-demand or continuous inkjet type, are generally not capable of precisely steering droplets to control the placement of droplets precisely within pixels areas of the printed image. In both drop-on-demand and continuous inkjet technologies, failure to accurately control print droplet placement within printed pixel areas reduces the image quality that could be achieved if such control were available. Thus it would be desirable to enable control of the placement of droplets precisely within pixels areas. In some cases, control of drop placement can be used to directly compensate nozzle manufacturing defects which result in drop placement errors, for example by using a lookup table in which manufacturing defects were quantified; in other cases, control of drop placement can be used to directly improve image quality even in the absence of drop placement errors. For example, improvements in image quality can be achieved by deliberately altering the positions of drops within printed pixel areas in an imagewise fashion when printing text. Such alterations can better replicate the intended positions of sharply defined image features such as curved portions of script fonts. Control of drop placement is useful in producing halftone images for graphic arts proofing.
As controlling drop placement has proven difficult, related technologies have been developed to improve image quality that do not require precise control of the positions of drops within printed pixel areas to improve the visual appearance of images. For example, the use of “multiple passes” or “banding passes” in inkjet printers averages out errors in print drop placement that may be inherent in any one nozzle by employing many different nozzles during multiple passes, as will be described. Also, software algorithms can be employed to improve image quality. However, these methods suffer from disadvantages of cost and complexity and the degree to which they improve image quality.
For example, in a printhead with an array of ink nozzles, individual nozzles, differing slightly in fabrication, cause errors in drop placement, either in the direction in which the print head is scanned (fast scan direction) or in the direction in which the receiving medium is periodically stepped (slow scan direction, usually orthogonal to the fast scan direction). For the most part, these minor differences result in placement errors no larger than some fraction of a pixel dimension. Nonetheless, under some conditions, small placement errors within this sub-pixel range of dimensions cause undesirable image artifacts known as banding, most noticeable in areas of text or areas of uniform color. To suppress banding, drop-on-demand inkjet printers in particular use multiple passes (so-called banding passes) in printing images, each banding pass using a different subset of nozzles on the printhead to eject drops. Nozzles are selected dependent on particular algorithms or are selected at random. Repetitive errors in drop placement can thereby be distributed spatially. For example, drops printed in two adjacent lines parallel to the scanning direction of the printhead (fast scan direction) would be printed by many nozzles, each subject to its own slight misdirection and consequent drop misplacement, so as to reduce repetitive misplacements. This technique introduces pseudo random spatial variations in drop position. Such positional “noise” in the printed drop, while itself an image artifact, is generally agreed to be preferred to the case of repetitive misdirection, which is more easily detected by the eye. The use of banding passes is effective even in cases in which misplacements of printed drops change unpredictably with time and/or do not arise from nozzle imperfections. For example, distortion of the media due to wet loading, can result in image artifacts due to misplacement of drops one to another and environmental factors such as mechanical vibrations in the printer or fluctuating air currents near the printhead can also result in image artifacts due to misplacement of drops. While multiple banding passes enable a printhead to correct for known banding errors, a more complex printing pattern is required as well as a more complex medium transport mechanism. The use of banding passes necessarily requires more time to print an image, since not all nozzles are used all the time. Under worst-case conditions, correction for band effects can result in significant loss of productivity, even as high as 10× by some estimates. It should be noted that most continuous inkjet printers do not have scanned printheads and hence cannot easily adapt approaches such as the use of banding passes common in drop-on-demand printers.
Conventional software methods, which do not necessarily reduce productivity, can also be applied to improve image quality. These well-known techniques include dither matrices, blue noise masking, FM screening, and error diffusion. For example, U.S. Pat. No. 5,726,772 entitled “Method and Apparatus for Halftone Rendering of a Gray Scale Image Using a Blue Noise Mask” to Parker et al. discloses the use of ordered dither algorithms using fixed-size threshold screen patterns. U.S. Pat. No. 5,875,287 entitled “Banding Noise Reduction for Clustered-Dot Dither” to Li et al. discloses an improved method for minimizing banding artifacts using offset dither matrices. U.S. Pat. No. 6,443,549 entitled “Continuous Tone Reproduction Using Improved Ink Jet Droplet Dispersion Techniques” to Bitticker et al. discloses a hybrid dot placement scheme using different types of dot dispersion, such as error diffusion and dither matrices, based on the overall density of an area of the image. As yet another approach, U.S. Pat. No. 5,937,145 entitled “Method and Apparatus for Improving Ink-Jet Print Quality Using a Jittered Print Mode” to Garboden et al. discloses the employment of “jittering” algorithms to vary droplet timing in a scanning inkjet printer of the drop-on-demand type. While the software solutions of these prior art methods are able to provide some measure of help for reducing banding and other image artifacts, there are limitations to these solutions and some room for improvement. Specifically, limitations of the print hardware constrain the level of adjustability to one or more full pixel-to-pixel distances, rather than allowing movement over a fraction of a pixel. Dither matrices, blue noise, and other techniques are limited by hardware-imposed constraints, such as the inability to control individual nozzles in a row or matrix. Therefore, these existing methods manipulate the image data before sending it to the printer in order to compensate for characteristics of the imaging system. Improvement of printer hardware performance itself, including methods to control drop placement within pixel areas could alleviate at least some of the need to implement these software solutions in many types of imaging applications.
It can be seen from the above discussion that the ability to accurately control print droplet placement within printed pixel areas could provide valuable alternatives to techniques currently used to improve image quality or to supplement those techniques when used in combination with them.
Some progress has been made in this regard in the case of continuous inkjet printing. For example, although early continuous ink jet printing technologies were not capable of steering droplets ejected from individual nozzles so as to accurately position printed drops within printed pixel areas, later continuous inkjet technologies were disclosed which provided methods for controlling the placement of droplets in both the slow scan and fast scan directions precisely within pixels areas of the printed image:
U.S. Pat. No. 4,347,521 (Teumer) discloses a print head employing a complex set of electrodes for droplet deflection in a continuous ink jet apparatus so that a plurality of inkjet nozzles are able to print in the same pixel area;
U.S. Pat. No. 4,384,296 (Torpey) similarly discloses a continuous ink jet print head having a complex arrangement of electrodes about each individual print nozzle for providing multiple print droplets from each individual ink jet nozzle;
U.S. Pat. No. 6,367,909 (Lean) discloses a continuous ink jet printing apparatus employing an arrangement of counter electrodes within a printing drum for correcting drop placement;
U.S. Pat. No. 6,517,197 (Hawkins et al.) discloses an apparatus and method for corrective drop steering in the slow scan direction for a continuous ink jet apparatus using a slow-scan droplet steering mechanism that employs a split heater element;
U.S. Pat. No. 6,079,821 (Chwalek et al.) discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and to deflect those ink droplets. A print head includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets whose trajectories can be controlled and non-printed ink droplets; and
U.S. Pat. No. 6,588,888 (Jeanmaire et al.) discloses a continuous ink jet printer capable of forming droplets of different size and with a droplet deflector system for providing a variable droplet deflection for printing and non-printing droplets.
While the above cited patents disclose methods for placing droplets precisely within pixel areas of the printed image in both the slow scan and fast scan directions, they require special nozzle designs and/or hardware which adds cost and complexity. Thus despite the cited improvements, technology for precisely controlling drop placement within pixel areas has not been commercialized due to cost and complexity. The capability of cost effectively providing precise control of drop placement in the fast scan direction, as described in commonly assigned copending U.S. application Ser. No. entitled “Continuous Inkjet Printer Having Adjustable Drop Placement” cost effectively affords partial control of droplet placement within pixel areas for continuous inkjet printers but provides only one-dimensional correction of droplet placement thereby allowing only a partial set of solutions for improving image quality.
Additionally, not all prior art solutions can be applied to a continuous ink jet printing apparatus, particularly for corrections in placement less than the center to center spacing of drops printed in succession and particularly where such an apparatus does not employ electrostatic forces for droplet deflection. Taken by themselves, none of these solutions meet all of the perceived requirements for robustness, sub-pixel placement accuracy, and cost. In particular, there remains significant room for improvement in controlling droplet placement in both orthogonal fast and slow scan directions. Specifically, there are advantages to a solution that would allow, at any position within a pixel area:
(a) control of the centroid of the printed drop anywhere within its associated pixel area;
(b) control of the number of droplets used to form a printed drop; and
(c) control of the spread of each printed drop.
Thus it can be appreciated that there is a continuing need for cost effective control capabilities for improved dot positioning for each ink jet nozzle in a continuous ink jet print head, particularly where these added capabilities can be used to suppress imaging artifacts.